Effects of side-effect risk framing strategies on COVID-19 vaccine intentions: a randomized controlled trial

  1. Nikkil Sudharsanan  Is a corresponding author
  2. Caterina Favaretti
  3. Violetta Hachaturyan
  4. Till Bärnighausen
  5. Alain Vandormael
  1. Assistant Professorship of Behavioral Science for Disease Prevention and Health Care,Technical University of Munich, Germany
  2. Heidelberg Institute of Global Health, Heidelberg University, Germany
7 figures, 7 tables and 1 additional file

Figures

CONSORT flow diagram.

Of the 9002 enrolled participants, 8998 individuals completed the survey (response rate = 99.96%).

Effect of the vaccine side-effect framing strategies on participants’ vaccine intentions and perceived safety.

N=8998 for strategies 1 and 2; strategy 3 is estimated only among those that received a comparison risk (N=5998). We estimated the main effects of each treatment strategy on participants’ vaccine intentions using logistic regression models and on the perceptions of vaccine safety using OLS regression models. Control means for strategies 1 and 2 are 65% and 67% for strategy 3. Error bars represent 95% confidence intervals.

Differences in the framing effects by country, age, and sex.

Coefficients are the interaction effect of the main heterogeneity characteristic with the indicator for each treatment strategy and are presented as coefficients on the log-odds scale from logistic regression models. Error bars represent 95% confidence intervals.

Appendix 1—figure 1
Screenshots of the online, Gorilla-based, experiment content for 2 of the 10 possible experimental combinations (A) no risk label, no control group; (B) risk label, comparison to motor-vehicle mortality, absolute comparison.
Appendix 1—figure 2
Detailed randomization diagram across the main study factors.

We have shown the randomization here with sample sizes referring to both countries combined; in practice, we conducted this procedure stratified by country such that each cell in the diagram has exactly 50% US and 50% UK observation.

Appendix 1—figure 3
Screenshots of each experimental arm for US participants.

Note that we have labeled the arms for the figure, but participants were not shown this label. Reference mortality information for motor-vehicle and COVID-19 fatalities were taken from the Centers for Disease Control and Prevention (Centers for Disease Control and Prevention, 2021b; Centers for Disease Control and Prevention, 2020).

Appendix 1—figure 4
Screenshots of each experimental arm for UK participants.

Note that we have labeled the arms for the figure, but participants were not shown this label. Reference mortality information for motor-vehicle fatalities were taken from the Department of Transportation (Department for Transport, 2020) and COVID-19 from the Public Health England (Public Health England, 2022).

Tables

Table 1
Demographic composition of the study samples.
United States(N=4502)United Kingdom(N=4496)
Gender
Male2,216 (49.2%)2,236 (49.7%)
Female2,205 (49.0%)2,216 (49.3%)
Other81 (1.8%)44 (1.0%)
Age
18–303,090 (68.6%)1,859 (41.4%)
30–451,077 (23.9%)1,531 (34.0%)
45–60268 (6.0%)776 (17.3%)
60+67 (1.5%)330 (7.3%)
Education
Less than secondary education142 (3.1%)498 (11.1%)
Completed secondary education987 (21.9%)1217 (27.1%)
Some college1786 (39.7%)1014 (22.5%)
Bachelor’s degree1061 (23.6%)1182 (26.3%)
More than bachelor’s degree526 (11.7)585 (13.0%)
Race*
White3607 (80.1%)3813 (84.8%)
Black/African357 (7.9%)139 (3.1%)
Asian441 (9.8%)316 (7.0%)
Other329 (7.3%)190 (4.2%)
Hispanic538 (11.9%)
  1. Notes: *Participants were allowed to choose multiple options if they identified as a member of multiple races. We used the race/ethnic categories used by the US Census Bureau and the UK Office for National Statistics when asking individuals to report their race and/or ethnicity.

Table 2
Balance table of baseline participant characteristics between the no risk label and risk label treatment arms.
No risk label (N=4496)Risk label(N=4502)p-Value
Female (n, %)2224 (49.5%)2197 (48.8%)0.542
Age (mean years, SD)31.7 (12.6)31.6 (12.7)0.623
Education (n, %)0.836
 Less than secondary education320 (7.1%)319 (7.1%)
 Completed secondary education1097 (24.4%)1107 (24.6%)
 Some college1392 (31.0%)1409 (31.3%)
 Bachelor’s degree1143 (25.4%)1100 (24.4%)
 More than bachelor’s degree544 (12.1%)567 (12.6%)
  1. Notes: p-Values for gender and education correspond to a chi-squared test and a two-sided t-test for age.

Table 3
Balance table of baseline participant characteristics between the no comparison, comparison with motor-vehicle mortality, and comparison with COVID-19 mortality treatment arms.
No comparison (N=3000)Comparison with motor-vehicle mortality(N=3002)Comparison with COVID-19 mortality(N=2996)p-Value
Female (n, %)1478 (49.3%)1452 (48.4%)1491 (49.8%)0.547
Age (mean years, SD)31.8 (12.8)31.4 (12.5)31.6 (12.7)0.516
Education (n, %)0.873
 Less than secondary education210 (7.0%)202 (6.7%)227 (7.6%)
 Completed secondary education717 (24.0%)744 (24.8%)743 (24.8%)
 Some college931 (31.0%)933 (31.1%)937 (31.2%)
 Bachelor’s degree769 (25.6%)753 (25.1%)721 (24.1%)
 More than bachelor’s degree373 (12.4%)370 (12.3%)368 (12.3%)
  1. Notes: p-Values for gender and education correspond to a chi-square test and an ANOVA-test for age.

Table 4
Balance table of baseline participant characteristics between the absolute risk and relative risk treatment arms.
Absolute risk (N=2997)Relative risk(N=3001)p-Value
Female (n, %)1473 (49.1%)1470 (49.0%)0.918
Age (mean years, SD)31.4 (12.4)31.6 (12.8)0.543
Education (n, %)0.897
 Less than secondary education222 (7.4%)207 (6.9%)
 Completed secondary education737 (24.6%)750 (25.0%)
 Some college934 (31.2%)936 (31.2%)
 Bachelor’s degree743 (24.8%)731 (24.3%)
 More than bachelor’s degree361 (12.0%)377 (12.6%)
  1. Notes: p-Values for gender and education correspond to a chi-square test and a two-sided t-test for age.

Table 5
Independent and combined effects of risk labeling and motor-vehicle mortality comparisons on willingness to take a hypothetical COVID-19 vaccine.
Effect size(percentage points)p-Value
Effect of risk labeling
(ref: no risk label)
N=8998
3.00.003
Effect of motor-vehicle comparison
(ref: no comparison)
N=8998
2.40.049
Effect of both risk labeling and a motor-vehicle comparison
(ref: no risk label nor comparison)
N=3002
6.1<0.001
  1. Notes: Outcome: ‘Would you take this vaccine?’ (yes = 1, others = 0). All the results are from logistic regression models with the results presented as average marginal effects. As per our pre-analysis plan, all regressions include covariates for age, sex, education, and country. Sample sizes for the relative to absolute comparison and effect of both labeling and a motor-vehicle mortality comparison are smaller since they are only estimated among subset of the total sample. p-Values are from two-tailed t-tests.

Appendix 1—table 1
Robustness of main paper results to alternative regression types and the inclusion/exclusion of covariates.
Logistic regression models with results presented as average marginal effectsLinear probability models
With covariates (main paper results)Without covariatesWith covariatesWithout covariates
Effect of labeled risk compared to unlabeled risk3.0 pp (p=0.003)3.0 pp (p=0.003)3.0 pp (p=0.003)3.0 pp (p=0.003)
Effect of comparison to motor-vehicle mortality compared to no comparison2.4 pp (p=0.049)2.4 pp (p=0.051)2.4 pp (p=0.0498)2.4 pp (p=0.052)
Effect of comparison to COVID-19 mortality compared to no comparison0.8 pp (p=0.496)0.7 pp (p=0.568)0.8 pp (p=0.497)0.7 pp (p=0.571)
Effect of relative comparison compared to absolute comparison1.3 pp (p=0.285)1.3 pp (p=0.277)1.3 pp (p=0.286)1.3 pp (p=0.277)
Effect of both risk labeling and a motor-vehicle comparison compared to no labeling or comparison6.1 pp (p<0.001)6.1 pp (p<0.001)6.1 pp (p<0.001)6.1 pp (p<0.001)
  1. Notes: Outcome: ‘Would you take this vaccine?’ (yes = 1, others = 0). As per our pre-analysis plan, covariates include age, sex, education, and country. Sample sizes for the relative to absolute comparison (N=5998) and effect of both labeling and a motor-vehicle mortality comparison (N=3002) are smaller since they are only estimated among subset of the total sample.

Appendix 1—table 2
Main paper results estimated using ordinal logistic regression models with a four-category outcome specification rather than logistic regression models with a binary outcome.
Odds ratio (p-value)
Effect of labeled risk compared to unlabeled risk1.15 (p=0.002)
Effect of comparison to motor-vehicle mortality compared to no comparison1.11 (p=0.050)
Effect of comparison to COVID-19 mortality compared to no comparison1.03 (p=0.541)
Effect of relative comparison compared to absolute comparison1.05 (p=0.327)
Effect of both risk labeling and a motor-vehicle comparison compared to no labeling or comparison1.31 (p<0.001)
  1. Note. Outcome: ‘Would you take this vaccine?’ (No, Unsure – leaning towards no, Unsure – leaning towards yes, Yes). As per the main paper results and pre-analysis plan, all regressions include controls for age, sex, country, and education. Sample sizes for the relative to absolute comparison (N=5998) and effect of both labeling and a motor-vehicle mortality comparison (N=3002) are smaller since they are only estimated among subset of the total sample.

Additional files

Download links

A two-part list of links to download the article, or parts of the article, in various formats.

Downloads (link to download the article as PDF)

Open citations (links to open the citations from this article in various online reference manager services)

Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)

  1. Nikkil Sudharsanan
  2. Caterina Favaretti
  3. Violetta Hachaturyan
  4. Till Bärnighausen
  5. Alain Vandormael
(2022)
Effects of side-effect risk framing strategies on COVID-19 vaccine intentions: a randomized controlled trial
eLife 11:e78765.
https://doi.org/10.7554/eLife.78765